Ballard Siphon Replacement Rebid

This project included construction of launch and retrieval shafts and 2,100 feet of 114-inch segmentally lined tunnel with an 85.5-inch diameter final liner. Additionally, two existing 36-inch wood stave pipelines were slip-lined with steel liners. Common when the siphon was originally constructed, the original pipes were similar to wooden barrels with metal compression bands holding the staves together. King County maintained flows through the aging pipes until the new pipeline was constructed. Once the new pipeline was installed and operational, the wood pipes were taken off-line, cleaned and slip-lined. The newly-lined wood pipes remain on the floor of the bay in case they need to be used in the future. The existing Ballard Regulator was expanded and reconfigured to divert flows through the new 85.5-inch line. An underground structure was constructed at the launch site that will direct flows from the new conduit to the existing North Interceptor.

To improve upon the schedule, JWF suggested use of a shaft excavation machine that was the first of its kind in North America. A Herrenknecht AG developed Vertical Shaft Machine (“VSM”) was used to build the 30-foot diameter, 145-foot deep launch shaft for the tunnel machine. The machine allowed crews to excavate the shaft in two months, which is three to four months faster than conventional excavation. Shaft building with the VSM involves excavating an initial section and building a concrete ring wall to support the machine, which is lifted with a crane and assembled in place. An operator remotely moves the machine down and precast concrete segments are installed to structurally line the shaft as the machine continues to excavate. The segments sink as the machine’s pivoting cutting arm digs out the soil, allowing new segments to be added from above.